Failed to get size of gamma for output default when trying to add new screen resolution

You don't need sudo to register the new mode with xrandr, try without sudo. Then you'll have to apply the new resolution with:

xrandr --addmode <your_connection_type> 1200x1000_60.00

Where <your_connection_type> is usually VGA1, DP1 or HDMI1. Check the output of xrandr to know the exact name of the connected output.

I tried almost all the answers with xrandr in English version, and I always have the same error message xrandr: Failed to get size of gamma for output default.

Then, there is one solution in Chinese saved my life, I just want to share that.

open the file

sudo nano /etc/default/grub

find the line

#GRUB_GFXMODE=640x480

edit 640x480 to your resolution eg: 1920x1080，并remove the #

for example:

GRUB_GFXMODE=1920x1080 

Update by the command

sudo update-grub

Then reboot your computer.

sudo reboot

Reference: https://blog.csdn.net/u013764485/article/details/78007370

If you have an nVidia or ATI GPU, have you tried changing your driver, either from proprietary to non-proprietary or vice versa?

My system:

~$ inxi -G
Graphics:  Card: NVIDIA C61 [GeForce 7025 / nForce 630a] 
           X.Org: 1.15.1 drivers: nouveau (unloaded: fbdev,vesa) Resolution: [email protected] 
           GLX Renderer: Gallium 0.4 on NV4C GLX Version: 2.1 Mesa 10.1.3

Depending on what kernel/distribution I am running, sometimes the proprietary nVidia 304 driver works, on others, e.g. Bodhi 3.0, with the same setup, can fail to read properly the size of the screen and also fail to read the gamma.

You can try to set your output with the following line, which will at least fix your error report, but may not fix your output:

xrandr --output default --gamma 0:0:0 --mode 1200x1000

You may also want to install the inxi utility and post the output of inxi -G here for more help.